There is a continuous effort to increase scanning capabilities of x-ray imaging systems. This is especially true in computed tomography (CT) imaging systems. Customers desire the ability to perform longer scans at increased power levels. The increase in scan times at higher power levels allows physicians to gather CT images and constructions in a matter of seconds rather than in a matter of several minutes as with previous CT imaging systems. Although the increase in imaging speed provides improved imaging capability, the increase causes new constraints and requirements for the functionality of the CT imaging systems.
A CT imaging system typically includes a gantry that rotates at various speeds in order to create a 360° image. The gantry contains an x-ray tube, which composes a large portion of the rotating gantry mass. The CT tube generates x-rays across a vacuum gap between a cathode and an anode. In order to generate the x-rays, a large voltage potential is created across the vacuum gap, which allows electrons to be emitted, in the form of an electron beam. The electron beam is emitted from the cathode to a target on the anode. In releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target, where they are abruptly slowed down to emit x-rays. The high voltage potential produces a large amount of heat within the x-ray tube, especially within the anode.
The vacuum vessel is typically enclosed in a casing filled with circulating cooling fluid, such as dielectric oil. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and the cathode. High temperatures at an interface between the vacuum vessel and a transmissive window in the casing cause the cooling fluid to boil, which may degrade the performance of the cooling fluid. Bubbles may form within the fluid and cause high voltage arcing across the fluid. The arcing degrades the insulating ability of the fluid. The bubbles can cause image artifacts that can result in low quality images.
Typically, a small portion of energy within the electron beam is converted into x-rays; the remaining electron beam energy is converted into thermal energy within the anode. Due to the inherent poor efficiency of x-ray production and the desire for increased x-ray flux, heat load is increased that must be dissipated. The thermal energy is radiated to other components within a vacuum vessel of the x-ray tube. Some of the thermal energy is removed from the vacuum vessel via the cooling fluid. Approximately 40% of the electrons within the electron beam are back-scattered from the anode and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result, the x-ray tube components are subjected to high thermal stresses that decrease component life and reliability of the x-ray tube.
Prior art cooling methods have primarily relied on quickly dissipating thermal energy by circulating coolant within structures contained in the vacuum vessel. The coolant fluid is often a special fluid for use within the vacuum vessel, as opposed to the cooling fluid that circulates about the external surface of the vacuum vessel.
As power of the x-ray tubes continues to increase, heat transfer rate to the coolant can exceed heat flux absorbing capabilities of the coolant. Other methods have been proposed to electromagnetically deflect the back-scattered electrons so that they do not impinge on the x-ray window. These approaches, however, do not provide for significant levels of energy storage a dissipation.
A thermal energy storage device or electron collector, coupled to an x-ray window, has been used to collect back-scattered electrons between the cathode and the anode. The electron collector is typically implemented in mono-polar x-ray tubes. The x-ray window is typically formed of a material having a low atomic number, such as beryllium. A significant amount of heat is generated from the impact of the back-scattered electrons on the electron collector and X-ray window, due to retention of a significant amount of kinetic energy in the back-scattered electrons.
In using the electron collector, the collector and window need to be properly cooled to prevent high temperature and thermal stresses, which can damage the window and joints between the window and collector. High temperature surfaces of the window and collector can induce boiling of the coolant. Bubbles generated from the boiling coolant can obscure the window and thereby compromise image quality. Extensive boiling of the coolant results in chemical breakdown of the coolant and the formation of sludge on the window, which also results in poor image quality.
Thus, there exists a need for an improved apparatus and method of cooling an x-ray tube window that allows for increased scanning speed and power, is relatively easy to manufacture, and minimizes blurring and artifacts in a reconstructed image.